Significant interest has been focused on the reduction of certain constituents in internal combustion engine exhaust. Recently, focus has been placed on diesel engines. Diesel engine exhaust typically contains gaseous emissions such as carbon dioxide (“CO2”), water vapor (“H2O”), unburned hydrocarbons (“HC”), carbon monoxide (“CO”), and oxides of nitrogen (“NOx”) including NO and NO2, along with solid and/or liquid condensed phase materials referred to as particulates. Treatment of diesel engine exhaust may involve various catalytic devices having one or more catalysts disposed on a substrate for reducing the levels of regulated constituents in the diesel exhaust. For example, diesel exhaust treatment systems may include an oxidation catalyst, also known as a diesel oxidation catalyst (“DOC”), to convert HC and CO to CO2 and water, a catalyst for the reduction of NOx, and a particulate filter, also known as a diesel particulate filter (“DPF”), for removal of particulates.
One diesel exhaust treatment technology of particular interest is the use of a selective catalytic reduction (“SCR”) catalyst for the reduction of NOx. This technology involves the catalytically-enhanced reduction of NO to nitrogen and oxygen by ammonia or an ammonia source such as urea. The efficiency of this reduction reaction is significantly impacted by the ratio of NO2:NOx in the exhaust stream entering the SCR reactor. The impact of this ratio on SCR efficiency is especially pronounced at lower operating temperatures (e.g., <300° C.). For a typical zeolite-based SCR catalyst, the desired NO2:NOx ratio is about 0.5, which is required for a fast SCR reaction. Exhaust coming out of an engine, however, often exhibits a less than ideal NO2:NOx ratio of less than 0.2. Fortunately, a DOC device, which is often placed upstream of an SCR reactor in diesel exhaust treatment systems, is capable of converting NO to NO2 so that the ratio of NO2:NOx in the exhaust stream entering the SCR reactor can more closely approach the desired ratio.
One issue, however, with reliance on a DOC device to convert NO to NO2 in order to enhance the NO:NO2 ratio in the exhaust stream entering the SCR device is that the NO to NO2 conversion efficiency of a DOC device can vary with aging and/or operating conditions. It would therefore be desirable to have the capability, on board of a vehicle, to monitor the NO to NO2 conversion efficiency of a DOC exhaust treatment device. Unfortunately, most NOx sensors are not capable of distinguishing between NO and NO2, so direct on-board measurement of the DOC's NO to NO2 conversion efficiency is not feasible. Accordingly, it is desirable to provide a system and method for measurement of a DOC exhaust treatment device's efficiency of converting NO to NO2.